cmdlib: check node stats in prereqs
cmdlib: check node stats in prereqs

This patch adds checks for offline nodes in most instance LUs so that we
can work with offline secondaries, but not with offline primaries. Some
cases (like grow disk, which needs both sides up) are not allowing
offline nodes at all.

Reviewed-by: ultrotter

bitemyapp/ganeti,apyrgio/ganeti,ganeti/ganeti,apyrgio/ganeti,grnet/snf-ganeti,leshchevds/ganeti,mbakke/ganeti,ganeti/ganeti,andir/ganeti,andir/ganeti,dimara/ganeti,onponomarev/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,grnet/snf-ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/snf-ganeti,mbakke/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,leshchevds/ganeti,yiannist/ganeti,dimara/ganeti,onponomarev/ganeti,bitemyapp/ganeti,andir/ganeti,yiannist/ganeti